Abstract
In this paper we completely describe functions generating the infinite totally nonnegative Hurwitz matrices. In particular, we generalize the well-known result by Asner and Kemperman on the total nonnegativity of the Hurwitz matrices of real stable polynomials. An alternative criterion for entire functions to generate a Pólya frequency sequence is also obtained. The results are based on a connection between a factorization of totally nonnegative matrices of the Hurwitz type and the expansion of Stieltjes meromorphic functions into Stieltjes continued fractions (regular \(C\)-fractions with positive coefficients).
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Notes
In general, the condition to be meromorphic is replaced by less restrictive \(F(\overline{z})=\overline{F(z)}\). Basic properties of \(\mathcal {R}\)-functions can be found, for example, in [14] and (for the meromorphic case) in [23]. In this paper we confine ourselves to meromorphic functions only.
The notation \([a]\) stands for the integer part of \(a\).
The separate convergence of numerators and denominators was shown by Śleszyński in [20].
In fact, even an estimate stronger than (2.17) is valid (cf. [20, p. 105]). For each tuple of distinct numbers \((i_1,i_2,\dots ,i_k)\) there is only one summand \(\Big (\sigma _{i_1}^{(j)}\sigma _{i_2}^{(j)}\cdots \sigma _{i_k}^{(j)}\Big )^{-1}\) in the right-hand side of (2.16). At the same time, the sum
$$\begin{aligned} \sum _{i_1=1}^\infty \sum _{i_2=1}^\infty \dots \sum _{i_k=1}^\infty \Big (\sigma _{i_1}^{(j)} \sigma _{i_2}^{(j)} \cdots \sigma _{i_k}^{(j)}\Big )^{-1} \end{aligned}$$contains exactly \(k!\) such summands. Therefore,
$$\begin{aligned} a_k^{(j)}< \frac{1}{k!} \left( a_1^{(j)}\right) ^{k} \text { for }k=2,3,4,\dots . \end{aligned}$$
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The author is grateful to Olga Holtz and Mikhail Tyaglov for helpful comments and stimulating discussions.
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Communicated by Daniel Aron Alpay.
This work was financially supported by the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement no. 259173.
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Dyachenko, A. Total Nonnegativity of Infinite Hurwitz Matrices of Entire and Meromorphic Functions. Complex Anal. Oper. Theory 8, 1097–1127 (2014). https://doi.org/10.1007/s11785-013-0344-0
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DOI: https://doi.org/10.1007/s11785-013-0344-0